It is standard practice in many extruder systems to inject a fluid into the plastified mass right in the extruder barrel. Such a fluid might be a gaseous or liquid foaming agent, a cross-linking agent, or the like. Only such fluids are thus injected which must be under pressure or which react with the normally thermoplastic mass in the barrel so that they have to be added as late as possible in the process.
The system of German published specification No. 1,909,211 controls the dosing of the fluid additive in accordance with the axial displacement of the worm of the extruder. In this manner the ratio of additive fluid to plastic resin is relatively fixed for a good-quality product. The main problem with this system is that the pressure of the plastic mass in the extruder varies considerably. It increases as the worm starts to rotate, then drops substantially, and falls substantially to zero when the worm stops rotating. Furthermore the pressure varies as the worm is axially displaced in systems using an axially displaceable worm. Thus the back pressure effective on the dosed fluid additive varies so that it is often difficult to maintain a fixed ratio between the fluid additive and the plastic resin.
German printed patent application No. 1,964,748 injects the additive fluid between pressure peaks into the extruder, that is between injection operations. Such a system still results in occasional excessive or inadequate dosing of the fluid additive, as the viscosity and pressure variations still lead to a varying back pressure effective against injection of the fluid additive.